The entire procedure usually takes 8-20 minutes depending on the number of eggs that have developed and the ease of accessing the ovaries. In many centers this is performed with local anesthesia and mild sedation, but some patients prefer to be asleep. Generally, the patient may leave after 30-60 minutes of observation. Once the eggs are harvested, they are isolated from the fluid that surrounds them (called follicular fluid) and placed in a special incubator where temperature, humidity and gas content are tightly controlled.
Phase 3: The third phase involves fertilization of the eggs. In most cases this is simply a process of preparing sperm from a specimen produced by the husband and placing sperm in the same dish as the egg(s). For others, a procedure called ICSI (mentioned below) is employed. About 24 hours later, the eggs are observed for changes suggesting that fertilization has taken place, mainly the presence of two early (pronuclear) masses within the egg.
What takes place over the next few days (in between phase 3 and 4) constitutes the weak link in the IVF process. Simply stated, the soup present in the woman’s tubes and uterus contains many substances which we have not identified to date -- let alone duplicated -- but that normally sustain an egg and early embryo in a woman's body. In natural fertilization, the fertilized egg enters the uterus about four days after ovulation and fertilization, and then does not physically implant in the uterine wall for another two days or so. Until recently, our laboratories could not sustain embryo development for this duration, so clinics were forced to place embryos in the uterus two or three days after egg retrieval.
For several reasons, an embryo is less likely to survive when placed in the uterus at this younger age. For example, the embryo's own genes do not take control until four cells have developed; before that, genetic messages come from the egg. Therefore, at three days, 20% to 30% of embryos which appear normal under the microscope are actually genetically imbalanced and will not survive after implantation. Therefore more embryos are generally implanted than are needed in the hope that one will survive. Unfortunately, this results in high multiple pregnancy rates; usually 25%—40% of those who conceive have multiple pregnancies, with triplets not unusual.
As our laboratories become better able to support embryo growth, we can hold an embryo in the lab until day 5-6. This improves results in two ways. First, because genetic abnormalities are much easier to detect by the fifth day, we end up with a population of embryos more likely to be genetically normal and capable of ongoing development. Second, we therefore need fewer embryos per implantation, reducing the multiple pregnancy rate while maintaining high pregnancy rates.
Sometimes the embryo needs additional help in the implantation process. Normally, when the embryo reaches the uterus it must break through its own outer membrane, called the zona pellucida, in order to implant. This is known as "hatching" (imagine a chicken hatching from an egg). When the embryo develops in the laboratory, the outer membrane may become thicker and harder than under normal conditions. This can impede the ability of the embryo to break through its wall and implant in the uterus. Sometimes we aid the embryo by purposefully weakening the membrane, either applying an acidic solution at one point, or making a small hole with a tiny glass needle or special laser (still experimental). This is called "assisted hatching." Authorities in the field do not universally accept assisted hatching as a truly beneficial aid, but some feel that it may be helpful, especially in women over 35.
Another technique used in IVF is "co-culture." Co-culture refers to adding live cells grown in tissue culture from the tube, uterus or sometimes kidney of human, primate, or bovine (cow) sources in order to supply hormones, growth factors and nutrients to the embryo while in the incubator. This approach is far from mainstream, and will probably be used less often as media continue to improve.